The dolphin is classified as sea animal (or sea ›monstrum‹) and not as a fish. This also happens in texts about fishing or selling food. As all peaceful animals (→ deer, → elephant, → camel and dolphin) it lives very long (up to 140 years). A reference to Mt 5, 10 is used to explain this.
The longest description of the dolphin is found in Der naturen bloeme, Jacob van Maerlants adaptation of Thomas of Cantimpré’s De natura rerum. The dolphin is the fastest sea animal. Dolphins are pregnant during 10 months. After birth the mother protects the young against the father but when the young is grown, the mother also attacks her child. They grow during 10 years. After ten months they have reached sexual maturity. They mate and bear during summer, never in winter. When they are angry their fins stand firmly, when they are afraid or tired the fins hang down. Dolphins love music. The story of Arion shows this clearly. They care for each other: if a dolphin dies the other dolphins bring the body to land because they do not want the fishes to eat of the corpse. If men have eaten a dolphin and they swim in the sea, other dolphins attack and kill those people. Otherwise, however, they are friendly to people and become easily ›domesticated‹.
In the Nile live dolphins with very sharp fins. These they use to kill crocodiles.
[I still have to consult the – not edited – Middle Dutch translation of De proprietatibus rerum].